This application is based on and hereby claims priority to German Application No. 102 08 482.3 filed on Feb. 27, 2002, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The invention relates to a coil system having an amplifier and having at least one coil which is electrically connected to the amplifier. The coil system is intended in particular for use in a magnetic resonance appliance.
2. Description of the Related Art
Magnetic resonance technology is a known technique for obtaining images of the interior of the body of an object that is being examined or investigated. In this case, gradient fields which are switched at high speed and are produced by a gradient coil system are superimposed on a static basic magnetic field, which is produced by a basic field magnet system, in a magnetic resonance appliance. Furthermore, the magnetic resonance appliance has a radio-frequency system which injects radio-frequency signals into the investigation object to produce magnetic resonance signals, and which receives the magnetic resonance signals that are produced and on whose basis magnetic resonance images are produced.
In this case, a high degree of homogeneity of the basic magnetic field is a critical factor for the quality of the magnetic resonance images. Inhomogeneities of the basic magnetic field within an imaging volume of the magnetic resonance appliance cause geometric distortion of the magnetic resonance image, with this distortion in proportion to the inhomogeneities. The field homogeneity is particularly important for so-called fast pulse sequences, for example for the echo planar process.
So-called shim systems are used to improve the basic magnetic field homogeneity within the imaging volume. In this case, a distinction is drawn between passive and active shim systems.
In a passive shim system, a number of iron plates are fitted arranged in a suitable manner in the examination area of the appliance. The basic magnetic field within the imaging volume is measured for this purpose before the iron plates are fitted. The measured values are used by a computation program to determine the suitable number and arrangement of the iron plates. A passive shim system with a specific calculation process for the number, thicknesses and positions of the individual iron shim plates is described, by way of example, in DE 19922652 C2.
In an active shim system, on the other hand, shim coils to which direct currents can be applied are used to homogenize the basic magnetic field. Operation of the shim coils requires power supply units which supply very constant and reproducibly adjustable direct currents. The active shim system is used, inter alia, for fine correction when a very high degree of homogeneity is required, for example in order to correct field distortion caused by the susceptibility of the object that is being examined and is at least partially located in the imaging volume. An active shim system with a specific adjustment process for the currents which are fed into at least one of the shim coils and gradient coils is described, by way of example, in DE 10030142 C1.
Furthermore, it is known from DE 19511791 C1 for linear inhomogeneity of the basic magnetic field, that is to say a first-order field disturbance, to be corrected by applying an offset current to a gradient coil. In this case, the offset current is a constant current, which is superimposed on a gradient coil current which carries out a gradient sequence. This approach is based on the idea that the basic magnetic field within the imaging volume can be described by coefficients of a spherical function series development. Higher-order field inhomogeneities are no longer compensated for by the gradient coils but by a number of shim coils which essentially each compensate for one of the spherical function coefficients. Nine shim coils are generally used for this method, so that, together with the three gradient coils, it is possible to compensate for the 12 spherical function coefficients which disturb the field homogeneity to the greatest extent.
The known methods and systems for improving the basic magnetic field homogeneity are either very complex to implement, as in the case of active shim systems, or, like passive shim systems, suffer from their compensating effect being dependent on the temperature. This is because the magnetic characteristic of the iron shim plates is dependent on temperature, so that the shim state and hence the basic magnetic field homogeneity vary with temperature.
An object of the invention is therefore to specify a system which is suitable for improving the basic magnetic field homogeneity, is easy to implement, and is also largely independent of temperature.
A coil system according to the invention has at least one amplifier, a coil which is electrically connected to the amplifier, as well as a current controller or current control means for adjusting the current which is supplied from the amplifier to the coil such that, at least on average, a predetermined power loss is converted to heat in the coil.
The invention is in this case based on the knowledge that the main heat input which leads to a temperature change in passive iron shim plates is produced by the current flow in the gradient coils. Depending on the sequence current which is flowing at any given time in one of the relevant gradient coils, the passive iron shim plates which are arranged adjacent to the gradient coils are either heated, or are cooled down again, for example during the sequence pauses. It has been found that the disturbing temperature influence on the passive iron shim plates can be overcome by applying additional current to the (gradient) coil. The additional current component which is fed into the coil is in this case of such a magnitude that the total current flowing overall in the coil is at least equal to the maximum sequence current that occurs during normal operation. The electrical power loss which is caused by the total current in the coil is therefore then also approximately equal to the power loss which would otherwise occur only when the maximum sequence current is flowing.
As a consequence of this, the coil also emits an approximately constant amount of heat, at least when averaged over time. After a thermal starting phase, which follows the magnetic resonance appliance being switched on, the iron shim plates which are arranged in the immediate vicinity are in a largely homogeneous temperature field. This overcomes the temperature-dependent fluctuations of the magnetic characteristics of the iron shim plates, and compensation is always provided in virtually the same way for the basic magnetic field throughout the actual operation period.
The current control means that are additionally provided and the application of an additional current component associated with this to the coil can thus be used firstly for the passive iron shim plates, which are less complex than the active shim coils, but while on the other hand also at the same time avoiding the otherwise normal temperature dependency of the compensation when using iron shim plates.
The fact that the additional heat is produced in precisely the same place as that in which the heat which is a function of time as a result of the sequence currents is also produced, namely in the coil itself, is also critical for advantageous effects. No significant additional components, for example in the form of separate heating elements or else separate power electronics, are thus required for the additional heating. Components which are present in any case are primarily used for the amplifier and the coil. This also considerably reduces the complexity level required for implementation. Furthermore, a conventional coil system can easily be retrofitted since, essentially, it is possible to use the existing components.
The current which is fed into the coil preferably contains a first component which is used in the coil to produce the graded field strength. This is a so-called pulse current or sequence current, which is repeated at cyclic intervals. The first current component is referred to as the sequence current component here. A second current component, which is referred to as the heating current component here, is also fed into the coil. The heating current component is not used for field production but for heating, and is of such a magnitude that the total amount of heat emitted from the coil corresponds to a predetermined value.
In particular, the heating current component has no mean value. For example, it may be in the form of a periodic alternating current signal or else, in particular, in the form of a sinusoidal alternating current signal. The critical factor for imaging with a magnetic resonance appliance is always the integral of the gradient field strength over time. It is therefore advantageous for the time integral of the heating current component to be averaged at least within the time period in which the sequence current component is not equal to zero. This avoids any undesirable effect of the heating current component influencing the field.
For the heating current component to have as little field producing effect as possible, it is advantageous for a frequency of the heating current component to be synchronized to a clock frequency which is present in any case, in particular in a magnetic resonance appliance. If the frequency of the heating current component and, in particular, the switching clock for the sequence current component are synchronized to one another, this is particularly advantageous in terms of the heating current component having a negligible mean value during the sequence period of the sequence current component.
The time integral, determined over the sequence period of the sequence current component, of the heating current component advantageously assumes the value zero. As far as possible, this should be true for all feasible sequence periods. This is because different sequence periods may be used during operation of a magnetic resonance appliance. In particular, the time integral should assume the value zero even for the shortest sequence period. The field which is produced by the sequence current component is then virtually uninfluenced by the heating current component.
It is likewise possible for the sequence and heating current components to be separated in time. In this case, the sequence current component assumes a value other than zero only during the sequence pauses. This makes it possible to virtually preclude any disturbing influence from the heating current component on the gradient field from the start. Such time decoupling of the two current components is particularly advantageous when the period duration of the sequence current component is shorter than the thermal time constant.
In one advantageous embodiment, the current controller is designed such that the heating current component is appropriately slaved to the sequence current component at that time, so that the total heating power components produced by the two current components are equal to a predetermined value. This predetermined value may, for example, represent the heating power produced by the highest possible sequence current.
It is also advantageous for the coil with a regulated current supply to be a z-gradient coil of a magnetic resonance appliance. Owing to the specific spatial arrangement, a z-gradient coil such as this results in a greater heat input than x-gradient and y-gradient coils. If the amount of heat that is input differs to a very major extent, it may therefore be sufficient to equip only the z-gradient coil with current control means for the additional application of a heating current component. This contributes to reducing the power that is drawn and, furthermore, to reducing the complexity.
It is advantageous to include a detector providing detection means for detecting the heat power which is emitted from the coil. In particular, a real power sensor is in this case used for detection of total real power that is fed into the coil, or else at least one temperature sensor may be used, which is fitted, by way of example, in the vicinity of iron shim plates. The heating current component can then be readjusted as appropriate on the basis of the information obtained by using the detection means.
In particular, a current regulation control loop is provided as a component of the current control means for this purpose.
In a further variant, the disturbance influence of the heat current component is further reduced by the coil conductor being subdivided into two strand elements. This can be achieved without any problems, especially if the coil contains a stranded conductor. One half of the individual strands is then joined together to form a first conductor element, and the second half is joined together to form a second conductor element. The two conductor elements formed in this way are advantageously arranged in a bifilar manner, so that the heating current component does not produce any field. Furthermore, the bifilar arrangement has very low self-inductance, so that a heating current component can be fed in even at a relatively high frequency without any problems. By way of example, a frequency of 10 MHz is used. The high heating frequency in this case results in more heating power than a lower heating frequency owing to the thinner skin depth.
The two conductor elements of the coil are preferably short-circuited to one another at one of their ends and are connected at their other respective end such that they can selectively be operated with short-circuited or disconnected second conductor ends. Short-circuited operation is intended in particular for the sequence current component while, on the other hand, operation with disconnected second conductor ends is intended for the heating current component. An example of circuitry which is suitable for this purpose has a bandstop filter which is matched to the frequency of the heating current component, a bandpass filter which is matched to the same frequency, as well as a differential transformer.
Preferred exemplary embodiments of the coil system will now be explained in more detail with reference to the drawing, but without these exemplary embodiments having any restrictive effect whatsoever. To make it clear, the drawing is not to scale, and certain features are illustrated schematically. In detail: